Merge transaction summary and cart into one screen

Requires: !488 (merged)

Covers some parts of #233. Closes: #109 (closed)

MR details:

• removed moving to dashboard binding
• added Broadcast and Save to file buttons on top with (still) function key bindings
• moved transaction metadata (now it's being updated every time that cart changes)
• enabled to enter cart without operations in transaction
• moved loading transaction from file to cart - it's now possible to edit it.
• added JSON button which shows dialog with raw json of chosen operation
• moved sign with key section to cart
• adjust tests to upper changes